Name resolver based on the c-ares library.

This is a non-blocking and non-threaded resolver.  It may not produce
the same results as the system resolver, but can be used for non-blocking
resolution when threads cannot be used.

c-ares fails to resolve some names when ``family`` is ``AF_UNSPEC``,
so it is only recommended for use in ``AF_INET`` (i.e. IPv4).  This is
the default for ``tornado.simple_httpclient``, but other libraries
may default to ``AF_UNSPEC``.

This class requires ``pycares`` version 4. Since this class is deprecated, it will not be
updated to support ``pycares`` version 5.

.. deprecated:: 6.2
   This class is deprecated and will be removed in Tornado 7.0. Use the default
   thread-based resolver instead.